June 2, 1954
P. v. CHENEY ETAL
3,135,638
PHOTOCHEMICAL SEMICONDUCTOR MESA FORMATION
Filed Oct. 27, 1960
H9. 1.
2 Sheets-Sheet 1
36
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6.
Preston V. Cheney,
John G. Quetsch, Jr.,
[NVE/VTORS‘.
BY.
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June 2, 1964
P. v. CHENEY ETAL
3,135,638
PHOTOCHEMICAL. SEMICONDUCTOR MESA FORMATION -
Filed Oct. 27, 1960
2 Sheets-Sheet 2
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38
Preston V. Cheney,
John G. Que’rsch, Jr.,
INVENTORS.
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ml
A TTOR/VEY.
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United States Patent 0
1CC
3,135,638
Patented June 2, 1964
1
2
‘for mesa formation and crystal device fabrication ac
PHOTQCHEMICAL SEMICONDUCTOR MESA
FORMATION
Preston V. Cheney, Costa Mesa, and John G. Quetsch,
Anaheim, Calif., assignors to Hughes Aircraft Com
3,135,638
cording to this invention.
The formation of suitable mesas on suitably doped
semiconductor crystal slices (hereinafter explained for
silicon semiconductor crystals) according to this inven
pany, Culver City, Calif., a corporation of Delaware
tion involves the formation, on a predetermined surface
of a silicon crystal, of a layer of ?lms of silicon oxide and
Filed Oct. 27, 1960, Ser. No. 71,859
8 Claims. (Cl. 156—-11)
chromium metal to serve as a crystal surface protective
mask during preferential etching of the silicon crystal
This invention relates to semiconductor device fabrica 10 adjacent the mask, thus producing under the mask a
tion, and more particularly to a photochemical method
mesa of silicon crystal material suitable for subsequent
for forming mesas, or plateaus, on semiconductor crystals.
device fabrication. Suitable silicon oxide layers may be
In semiconductor device fabrication, it is desirable to
formed by oxidizing the surface of silicon semiconductor
produce sharply de?ned raised portions, or mesas, on
or by decomposing chemicals, such as silanes,
semiconductor surfaces. Particularly in transistor fab 15 crystals,
on a semiconductor such as silicon or germanium. It is
rication, which will be used ‘herein to illustrate this in
vention, sharply de?ned mesas will ‘allow the use of
presumed that such ?lms are silicon dioxide, and its prop
erties appear to be those of silicon dioxide; whether sili~
smaller area mesas for emitter and base portions of a
con monoxide is also present in such ?lms is not known.
transistor with equal or improved yield in production
Such ?lms are ‘hereinafter generally referred to as $10;
and equal or improved device characteristics and a smaller 20 ?lms.
range of device parameters in a given production run.
The layer of ?lms is formed by successively forming
In diode production, sharply de?ned mesas also allow
?lms of SiO2, chromium metal, and photosensitive polym
smaller ‘and more uniform junction areas to be made.
‘It is accordingly an object of this invention to produce
erizable (PSP) material, followed by photochemically
polymerizing selected portions of the PSP material, re
smaller, and more sharply de?ned mesas on semiconduc 25 moving unpolymerized portions thereof, selectively re
tor bodies; and more particularly to produce transistors
moving exposed (or uncovered) portions of the chromi
having more sharply de?ned mesas including the emitter
um ?lm, and thereafter of the SiO2 ?lm, to produce on
and base portions of the transistor, and to produce diodes
the surface layers of SiOz and chromium ?lms under
having more sharply de?ned mesas including a P-N junc
the polymerized PSP material.
tion.
30
The polymerized PSP material is preferably removed
This invention will be particularly explained with re
before
the etching of the silicon, and may be removed
spect to silicon semiconductor bodies, although it is ap
before the SiO, etching step so that the etching of the
plicable to germanium and other semiconductor mate
Si02 and of the silicon may proceed successively as a
rials.
single process step.
Silicon mesas have heretofore been formed by securing 35
As illustrated in the drawings, FIG. 1 shows a P-type
physical shields to silicon crystal surfaces and then sand
silicon semiconductor crystal 30 upon which a silicon
blasting and subsequently etching the damaged portions
oxide mask is to be formed for selective diffusion of
of the crystal formed during sandblasting. The physical
boron as a dopant, or electrical conductivity type deter
shields have been attached, for example, by using thermo
mining impurity. Various cleaning and degreasing steps
plastic material to fasten platinum pieces to the silicon 40 which are well known in semiconductor operations are
crystal surface. Platinum is sufficiently malleable ‘and
resistant to sandblasting and to silicon etches to protect
portions of the silicon from crystal damage and subse
quent removal. This method is limited to shields of sizes
omitted herein for clarity of presentation. The crystal 30
is subjected to an atmosphere of AS203 in argon at about
1200° C., to indiffuse arsenic and form an N-type region
311 on the crystal surface. The balance of the crystal 30
will be a P-type region 32. The above atmosphere will
simultaneously grow a silicon oxide (SiO2) ?lm 33 on
and shapes that can be physically produced and handled,
and is not easily susceptible to mechanization.
Silicon oxide ?lms have beenproposed for protection
the crystal, which is preferably enhanced by humidifying
of semiconductor surface areas during selective etching
the gas to about a 30° C. dew point before the end of
of unprotected portions of the semiconductor. Photo
the diffusion step. If desired, oxygen ‘gas may also be
chemical techniques for de?ning the silicon oxide areas 50 added to the atmosphere to promote formation of the
have been unsatisfactory due to poor de?nition of the
SiOz ?lm. The arsenic indiifusion step is preferably
silicon oxide areas andthe inability to secure adequate
maintained for 4 to 6 hours, by way of example, to pro
protection of the silicon oxide surface by photochemical
duce an N-type region of 4 to 6 microns depth and a sur
materials during etching of the oxide to de?ne the de
face
8:102 ?lm '33‘ of 0.4 to 0.6 micron, as shown in FIG.
55
sired oxide area.
'
2. Proportions have been exaggerated in the drawing for
It is a further object and advantage of this invention
illustrative purposes.
to produce semiconductor crystal shields by use of photo
A germanium ?lm 34 is next formed on the SiO2 ?lm
chemical process, and to utilize the same to produce
rnesas of sharply de?ned con?guration and of sizes and
shapes independent of mechanical handling techniques,
by evaporation of germanium in a vacuum furnace. Films
of from 0.15 to 9.0 microns have been used successfully,
but 0.4 to 0.6 micron gives effective coverage and uni
and by processes adaptable to automation.
formity. This ?lm is preferably formed by laying ger
The above and other objects and advantages of this
manium on a tungsten ?lament, and heating the ?lament
invention will be apparent from the balance of this spec
to evaporate the germanium in a vacuum of about 5 X10-5
:i?cation, disclosing the preferred embodiment of the in
mm. Hg to deposit the germanium on an exposed surface
65
vention, and in the accompanying drawings and claims
of the crystal at about 550° C. having the'SiOz ?lm
forming a part thereof.
thereon. FIG. 3 shows the resulting crystal 30 having
In the drawings:
an N-type region 31, a P-type region 32 for the balance
FIGS. 1 to 111 illustrate a sequential process for pro
of the crystal, an SiO2 ?lm 33 on the surface of the N-type
ducing in a silicon semiconductor crystal surface-adja
region, and a germanium metal ?lm 34 on the oxide ?lm.
cent regions of opposite electrical conductivity type suit
A ?lm 35 of photosensitive polymerizable (PSP) ma
able for mesa~type transistor fabrication.
terial, such as polyvinyl alcohol, or a product well known
FIGS. 12 to 2.1 illustrate a further sequential process
on the market and sold under the trade name of Kodak
3,135,638
Q
4i
Photo Resist by Eastman Kodak Company and believed
tern of square areas 41 of polymerized PSP material
to be a resinous ester of maleic anhydride and alkoxy
hydroxy acetophenone, is next formed on the surface
of the germanium, as shown in FIG. 4. The germanium
with the previous ?lm stripes 36.
is preferably lightly etched in a 4% hydrochloric acid
etch for 30 seconds to remove any germanium oxide which
may be present, and to improve adhesion thereto of the
PSP material ?lm. This ?lm may be painted on, sprayed,
or applied in any suitable way.
The ?lm 35 of PSP
material is dried at about 70° C., then selectively ex
posed to ultraviolet light, preferably through a photo
graphic ?lm mask, to polymerize portions 36 of the ?lm
graphic ?lm mask, to polymerize portions 36 of the
bridging the P and N regions of the crystal surface. The
material of the squares 41 is then baked and hardened as
The chromium ?lm now uncovered between the squares
41 is next removed by an HCl etchant of about 50% and
at about 70° to 80° C., to expose the underlying SiOz
?lm as shown in FIG. 16. This exposed portion of the
SiOz ?lm, and the silicon crystal thereunder, is then etched,
with a (1:122) solution of nitric acid, hydrofluoric acid
and acetic acid, for example, to remove material down
to and into the lower P-type crystal region 32, leaving
?lm 35 which it is desired to retain as shown in FIG. 5.
mesas of crystal under each square 41 as shown in FIG.
18.
terial 35, leaving areas, for example, stripes, of polym
a chromium solvent such as 37 to 39% HCl at 50° to 60°
hydrofluoric acid etch which selectively removes Si02
in the presence of silicon, germanium, and PSP material.
FIG. 8 shows the resulting structure with alternate ex
posed areas of silicon crystal and stripes of layered PSP
such material, provides sharp delineation of the mesa area
and adequate protection of the mesa area, and thus makes
The square layers of polymerized PSP material and
The crystal is then developed by rinsing with a solvent, 15
chromium, are next removed in successive etching opera
such as methyl ethyl ketone, trichloroethylene, or Kodak
tions utilizing a PSP material solvent such as trichloro
Photo Resist Developer sold by Eastman Kodak Com
ethylene, with brushing of the softened PSP material, and
pany, for the unexposed, hence unpolymerized, PSP ma
erized PSP material 36 as shown in FIG. 6. The crystal 20 C. The resulting structure, shown in FIG. 19, is next pro
vided with an ohmic contact on the reverse crystal face,
is then baked at 70° C. to further polymerize and harden
as by fusing aluminum 42 thereto as shown in FIG. 20
the ?lm portions 35.
for subsequent attachment of a collector. The crystal is
The exposed germanium between stripes of PSP ma
then diced to separate out crystal elements 43 as shown
terial 36 is next etched and removed, in an etchant such
in FIG. 21, each having a mesa structure thereon pro
as hydrogen peroxide and oxalic acid, to expose SiOZ be
tected by an oxide ?lm, for subsequent device fabrication
tween the stripes of PSP material 36 as shown in FIG. 7.
by removal of the oxide ?lm, attachment of leads and en
This etchant evolves relatively little gas when used below
capsulation.
40° C., and appears to have no substantial deleterious
The chromium metal ?lm 39, utilized in the mesa for
e?'ect on the PSP material ?lm.
The exposed SiOz is next etched and removed by a 30 mation steps to protect the crystal surface under the
material, germanium and SiO2.
The PSP material 36 is next removed by softening with
squares 41 of polymerized PSP material, together with
possible use of smaller mesa formations, more accurate
35 positioning and register of the mesas with the preformed
P- and N-type region surface junction within the mesa,
and many design variations, in a method which is highly
an appropriate solvent, such as methyl ethyl ketone, ace
suitable for mechanization and whose use improves de
tone, or trichloroethylene, and subsequent brushing, to ex
vice characteristics and reduces device rejects at the same
pose the germanium ?lm 34 in stripes as shown in FIG. 9.
The crystal 30 is then subjected to a germanium solvent 40 time. The silicon oxide ?lm serves both to separate the
chromium from the underlying semiconductor material
etch, such as hydrogen peroxide and oxalic acid, to ex
and to insure adequate adhesion of the chromium, par
pose the Si02 stripes on the crystal surface as shown in
ticularly in silicon semiconductors since chromium has
FIG. 10.
very poor adhesion to silicon material.
The above Si02 stripes are next used as a mask in a
The process as above illustrated depends upon the
boron diffusion process wherein the crystal surface is
selectivity of etchants used as well as the adherence and
exposed to a boron containing gas, such as boron oxide,
protective properties of the intermediate metal ?lm. It
to di?fuse boron into the crystal between the oxide stripes
may be applied to silicon and germanium and other
and convert the adjacent crystal region 38 to P con
semiconductor materials, such as III-V compound semi
ductivity type, as shown in FIG. 11.
conductors, when the second, or metal, ?lm is chromium,
An oxide ?lm 33A is then regrown on the crystal sur
nickel, platinum, molybdenum, palladium, alloys thereof
face, as shown in FIG. 12, exposure to about 30° C.
such as nickel-chromium alloys, and such other metals
dew point argon atmosphere at about 900° C. for 16 hours
as may be selectively etched in the presence of the silicon
being su?icient for the purpose. An oxide ?lm of about
oxide ?lm and the semiconductor, and which selectively
6,000 Angstroms thickness has been found sufficient to
insure adhesion of a subsequently deposited chromium 55 resists silicon oxide etchants and the semiconductor mate
rial etchants.
?lm. In some cases, especially where the oxide ?lm 33
Having disclosed our invention, we claim:
was relatively thick, it is preferable to remove the oxide
1. A process for producing a mesa formation on a
stripe 33 before growing the new ?lm 33A to insure
oxide adherence to the parent crystal.
A ?lm 39 of chromium as shown in FIG. 13 is then
vapor deposited on the oxide ?lm 33A, preferably by
germanium body, which comprises: forming a ?rst ?lm
of silicon oxide on a surface of said body; forming a sec
ond ?lm of a metal which may be selectively etched in
vaporizing chromium from a tungsten ?lament in a
vacuum furnace. The ?lm 39 of chromium is then cov
the presence of germanium and silicon oxide, and which
ered with a ?lm 40 of unpolymerized PSP material, which
on said ?rst ?lm; forming a third ?lm of photosensitive
polymerizable material on said second ?lm; exposing a
may be painted on with a brush, to produce the structure
of FIG. 14.
The PSP material of ?lm 40 is next exposed to ultra
violet light through a photographic ?lm mask positioned
to expose and polymerize squares 41 bridging the junc
tions between the P-regions 38 and the N-regions 31 at
the surface of the crystal, underlying the several ?lms,
as shown in FIG. 15. The PSP material of ?lm 40 is
then rinsed with a solvent for the unpolymerized PSP
material, such as methyl ethyl ketone, as before discussed,
to remove unpolymerized PSP material and leave the pat
selectively resists germanium and silicon oxide etchants,
portion of said third ?lm to suf?cient light to polymerize
said portion; developing said third ?lm to remove the
unpolymerized portion of said third ?lm; selectively re
moving the portion of said second ?lm uncovered by
removing said unpolymerized third ?lm portion; removing
the portion of said ?rst ?lm uncovered by removing said
portion of said metal ?lm; and removing a portion of
the germanium body adjacent the surface thereof un
covered by removing said portion of said ?rst ?lm, where
by to form a mesa on said germanium body.
3,135,638
5
2. A process for producing a mesa formation on a semi
conductor crystal body, which comprises: forming a ?rst
?lm of silicon oxide on a surface of said body; forming
a second ?lm of chromium on said ?rst ?lm; forming
a third ?lm of photosensitive polymerizable material on
a metal which may be selectively etched in the presence
of silicon oxide and said body, and which selectively re
sists etchants for said body and silicon oxide, on said
?rst ?lm; forming a third ?lm of photosensitive polym
erizable material on said second ?lm; exposing a por
said chromium ?lm; exposing a portion of said third
?lm to su?’icient light to polymerize said portion; devel
tion of said third ?lm to su?icient light to polymerize
said portion; developing said third ?lm to remove the
oping the same to selectively remove the unpolymerized
unpolymerized portion of said third ?lm; selectively re
portion of said third ?lm; selectively removing the por
moving the portion of the second ?lm uncovered by
tion of said chromium ?lm unprotected by said polymer 10 removing said unpolymeirzed third ?lm portion; remov
ized ?lm portion; selectively removing the portion of
ing the portion of said ?rst ?lm uncovered by removing
said oxide ?lm unprotected by said chromium ?lm; and
said portion of said second ?lm; and removing from
selectively removing a portion of the semiconductor
said body, adjacent said surface and uncovered by re
crystal uncovered by removal of said portions of chromium
moving said portions of said ?rst and second ?lms, a
and oxide ?lms whereby to produce said mesa formation. 15 portion of said body extending through the said PN
3. The process according to claim 2 wherein the body
junction.
is silicon.
7. The process according to claim 6 wherein the second
4. A process for producing a mesa formation on a
?lm is a metal of the class consisting of chromium, nickel,
body, which comprises: forming a ?rst ?lm of silicon oxide
platinum, molybdenum and palladium and alloys thereof.
on a surface of said body; forming a second ?lm of a 20
of said body and silicon oxide, and which selectively
8. A process for producing a semiconductor transistor
from a semiconductor body having a ?rst conductivity
type region adjacent a surface of said body, a second con
resists etchants for silicon oxide and said body on said
ductivity type region of opposite type to said ?rst region
metal which may be selectively etched in the presence
?rst ?lm; forming a third ?lm of photosensitive polym
adjacent and underlying said ?rst region to form a PN
erizable material on said second ?lm; exposing a portion 25 junction in part substantially parallel to said surface
of said third ?lm to suf?cient light to polymerize said por
and in part intersecting said surface, and a third conditiv
ity type region underlying said ?rst and second regions
tion; developing the same to selectively remove the un
and of the conductivity type opposite to that of the second
polymerized portion of said third ?lm; selectively re
moving the portion of said second ?lm uncovered by re
region to form a second PN junction therebetween, which
moval of said unpolymerized portion; selectively remov
comprises: forming a ?rst ?lm of silicon oxide on said
ing the portion of said oxide ?lm unprotected by said
surface; forming a second ?lm of a metal which may be
selectively etched in the presence of silicon oxide and said
second ?lm; and selectively removing a portion of the
body, and which selectively resists etchants for said body
body uncovered by removal of said portions of the sec
ond and ?rst ?lms whereby to produce said mesa forma
and silicon oxide, on said ?rst ?lm; forming a third ?lm
5. A process for producing a mesa formation on a
of photosensitive polymerizable material on said second
second ?lm; exposing a portion of said third ?lm to suf
silicon body, which comprises: oxidizing a surface of said
?cient light to polymerize said portion; developing said
tion.
third ?lm to remove the unpolymerized portion of said
body to form a silicon oxide ?lm thereon; forming a sec
third ?lm; selectively removing the portion of the second
ond ?lm of a metal which may be selectively etched in
the presence of silicon and silicon oxide, and which selec 40 ?lm uncovered by removing said unpolymerized third
tively resists silicon and silicon oxide etchant, on said
?lm portion; removing the portion of said ?rst ?lm un
?rst ?lm; forming a third ?lm of photosensitive polym
covered by removing said portion of said second ?lm;
and removing from said body adjacent said surface and
erizable material on said‘ second ?lm; exposing a portion
uncovered by removing said portion of said ?rst and
of said third ?lm to sui?cient light to polymerize said
portion; developing said'third ?lm to remove the unpo 45 second ?lms, a portion of said body extending through
lymerized portion of said third ?lm; selectively removing
the portion of said second ?lm uncovered by removing
said unpolymerized third ?lm portion; removing the por
tion of said ?rst ?lm uncovered by removing said por
tion of said metal ?lm; and removing a portion of the
silicon body adjacent the surface thereof uncovered by
removing said portion of said ?rst ?lm, whereby to form
a mesa on said silicon body.
6. A process for producing a semiconductor diode from
a semiconductor body having alternate regions therein of 55
P-type and N-type electrical conductivity forming a
PN junction in a plane substantially parallel to a surface
of said body, which comprises: forming a ?rst ?lm of
silicon oxide on said surface; forming a second ?lm of
the second PN junction.
References Cited in the ?le of this patent
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McFarland ___________ __ May
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Atalla et al ____________ __ Aug. 11,
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